Photographic film quality is directly related to the grain properties of the silver halide emulsion. Grain properties affect sharpness, granularity, chemical and spectral sensitization, pressure sensitivity, contrast, speed, developability, and other characteristics of the film. Silver halide grains having cubic, octahedral, cubooctahedral and tabular forms are all well known and have been used in photosensitive emulsions.
Single and double twinning has been known to occur in a number of known silver halide crystal shapes. See generally James, The Theory of the Photographic Process, 4th Ed., pages 21-22. Berry et al., Photographic Science and Engineering, Vol. 6, No. 3, June 1962 pages 159-165 describe doubly twinned cubic grains and speculate as to the existence of doubly-twinned cubooctahedral grains (at page 162). As illustrated in FIG. 1.9 of James, known double-twinned tabular grains have a ridge-trough edge structure.
Many methods have been proposed for producing thin tabular grains of intermediate or high aspect ratio. See, for example, Takada et al., U.S. Pat. No. 4,783,398 issued Nov. 8, 1988, wherein a growth modifier is used during nucleation and growth to produce a tabular grain having a 50-90 mole % content of chloride and an aspect ratio between 2:1 and 10:1. Maskasky, U.S. Pat. No. 4,400,463 issued Aug. 23, 1983, describes hexagonal and dodecahedral tabular grains with ridge-trough edge structures. Maskasky U.S. Pat. No. 4,713,323 issued Dec. 15, 1987 uses a large excess of chloride (0.5 molar CaCl.sub.2) and a growth modifier at nucleation and during growth to provide tabular grains with aspect ratios of 8:1 to greater than 12:1. In Daubendiek et al., U.S. Pat. No. 4,914,014 issued Apr. 3, 1990, a thin tabular grain silver bromide or bromoiodide emulsion is precipitated using excess bromide at the nucleation stage. A large stoichiometic excess of bromide is also recommended in Wilgus U.S. Pat. No. 4,434,226 and Kofron U.S. Pat. No. 4,439,520.
Double-twinned tabular grains have been prepared in a variety of forms. See, for example, Saitou U.S. Pat. No. 4,945,037, which describes tabular grains wherein the center and outer portions of the grain contain different mole percent amounts of iodide. Grains having both 1.1.1 and 1.0.0 planes are mentioned; see also Konica European Patent Publication Nos. 421,740 and 421,426. A commonly-assigned application by Jagannathan et al. entitled HIGH EDGE CUBICITY TABULAR GRAIN EMULSIONS describes AgBr and AgBrI tabular grains wherein less than 75% of the edge surfaces lie in 1.1.1 crystallographic planes.
A variety of methods for preparing photographic emulsions have involved using two or more different halide salts. Saleck et al., U.S. Pat. No. 4,075,020 issued Feb. 21, 1978, describes a halide converted emulsion made by continuous conversion of a more soluble silver halide into a less soluble silver halide. Finnicum et al., U.S. Pat. No. 4,147,551 issued Apr. 3, 1979, discloses a halide converted emulsion process which produces cubic and mixed crystal silver halide grains. Typical halide conversions produce amorphous grains and are unable to produce the more preferred tabular grains which exhibit superior photographic qualities. Saleck et al., U.S. Pat. No. 4,241,173 issued Dec. 23, 1980, reports a process where silver halide is precipitated in a large (50 mole %) excess of chloride under equilibrium conditions and without a separate nucleation step.
Despite these recent advances, a need remains for methods capable of controlling the aspect ratio of silver halide tabular grains, and also for preparing new twinned grain shapes. The present invention addresses these needs.